1. Field of the Invention
The present invention relates to a display apparatus and a display method, and a liquid crystal driver circuit and a liquid crystal driving method, and more particularly to a display apparatus and a display method, and a liquid crystal driver circuit and a liquid crystal driving method, suitable for displaying information by using cholesteric liquid crystal.
2. Description of the Related Art
A liquid crystal display apparatus utilizes, for example, TN (Twisted Nematic) liquid crystal and STN (Super Twisted Nematic) liquid crystal of a simple matrix type, TFT (Thin Film Transistor) liquid crystal and MIM (Metal In Metal) liquid crystal of an active matrix type.
In the simple matrix type, X electrodes and Y electrodes are disposed in a matrix shape and these electrodes are turned ON/OFF at proper timings to drive liquid crystal in a cross portion. A liquid crystal display apparatus of the simple matrix type is generally lower in price than a product using the active matrix type, because of easy manufacture and a high yield resulting from a small number of electrodes and simple structure. However, since the electrode of a liquid crystal constituting a pixel is not independent, there is voltage interference and nearby cells are influenced so that each pixel is difficult to be displayed clearly. On the other hand, as different from the simple matrix type, the active matrix type switches between on and off for each pixel (an active element is added to each pixel to drive liquid crystal). As compared to the simple matrix type, although the active matrix type is excellent in the performances such as a faster response time, a small after-image and a broad angle of visibility, its manufacture cost is high.
In order to retain displaying information on a display apparatus utilizing the above-described liquid crystal, it is necessary to continue to apply voltage to the liquid crystal. As voltage is applied to the liquid crystal for a predetermined time, an after-image phenomenon called “burn-in” occurs. In order to prevent the burn-in, for example, frame inversion techniques are used which invert the voltage to be applied to a pixel electrode, at a predetermined period. If polarity inversion techniques such as frame inversion are adopted, an amplitude of voltage to be applied to a signal line is required to be twice as high as that of a monopolar drive. Common inversion techniques or the like are used in order to halve the voltage amplitude to be applied to the signal line.
In contrast to the liquid crystal display apparatus described above, in a liquid crystal display apparatus using cholesteric liquid crystal, the state transits (between a planar state and a focalconic state) depending upon an applied voltage. By using this, information can be displayed and the information once displayed can be retained without supplying a power (e.g., see “Liquid Crystal Device Handbook”, published by the Nikkan Kogyo Shimbun, Ltd., Sep. 29, 1989, pp. 352 to 355).
Cholesteric liquid crystal selectively reflects light having a wavelength corresponding to a pitch of liquid crystal spiral layers in the planar state and becomes almost transparent in the focalconic state.
With reference to FIGS. 1 and 2, the structure of a cholesteric liquid crystal panel 1 will be described. FIG. 1 is a cross sectional view of a cholesteric liquid crystal panel 1, and FIG. 2 is a diagram illustrating the structure of two electrodes of the cholesteric liquid crystal panel 1.
Transparent column electrodes (ITO: Indium Tin Oxide) 12 are disposed in a stripe shape on a glass substrate 11-1 by vapor deposition (or sputtering), whereas transparent row electrodes (ITO: Indium Tin Oxide) 15 are disposed in a stripe shape on a glass substrate 11-1 by vapor deposition (or sputtering). Polyimide layers 13-1 and 13-2 of about several μm in thickness are disposed on the side of the glass substrates 11-1 and 11-2 where the transparent column electrodes 12 and transparent row electrodes 15 are vapor-deposited (or sputtered).
The glass substrates 11-1 and 11-2 are adhered together by a gap member or the like at a gap thickness of several μm (e.g., about 5 μm) in such a manner that the stripes of the transparent column electrodes 12 cross and face the stripes of the transparent row electrodes 15 via the polyimide layers 13-1 and 13-2. Cholesteric liquid crystal is injected into the gap between the glass substrates 11-1 and 11-2, for example, by a vacuum injection method to form a cholesteric liquid crystal film 14.
It is not necessary for the cholesteric liquid crystal panel 1 to orientate the polyimide layers and to mount a polarizing plate on the glass substrate, as in the case of generally used TN (Twisted Nematic) liquid crystal.
A molecular structure of cholesteric liquid crystal is a special helical structure (spiral structure). Since the helical structure changes with the value of an applied bipolar pulse voltage, the state changes. As shown in FIG. 3, cholesteric liquid crystal can take two stable states of a focalconic state and a planar state, depending on the value of an applied bipolar pulse voltage. The planar state is the state that makes a specific wavelength range of light be subjected to interference scattering, and the focalconic state is the state that light is transmitted over a broad range.
Information can therefore be displayed on the cholesteric liquid crystal panel 1 in a first color determined by a wavelength range in which light is reflected in the planar state and a second color viewed through the liquid crystal display when liquid crystal is transparent in the focalconic state. Namely, for example, a monotone of a specific wavelength color and a black color can be displayed on the cholesteric liquid crystal panel 1 by making cholesteric liquid crystal irregularly reflect light in the specific wavelength range in the planar state and coloring a portion under the cholesteric liquid crystal layer 14 in black and making the black color to be transmitted and viewed in the focalconic state.
As shown in FIG. 3, a voltage Vps of a bipolar pulse voltage necessary for changing the state of cholesteric liquid crystal to the planar state is approximately a twofold of a voltage Vfs of a bipolar pulse voltage necessary for changing the state to the focalconic state.
As a bipolar pulse voltage is applied to a predetermined pixel electrode, the cholesteric liquid crystal takes the focalconic state or the planar state, and if a voltage is not applied thereafter, the state is maintained. As a bipolar voltage pulse is applied again if necessary, the cholesteric liquid crystal can change its state in accordance with the applied voltage value. Namely, the cholesteric liquid crystal panel 1 using cholesteric liquid crystal can retain the information displayed upon application of a bipolar voltage pulse, without being supplied with a power thereafter.
FIG. 4 shows examples of a drive voltage waveform to be applied to a pixel electrode when a display of a predetermined pixel of the cholesteric liquid crystal panel 1 is to be changed. If a bipolar pulse having a voltage Vps is applied to a predetermined pixel electrode in the focalconic state, the state is changed to the planar state so that the display color is the first color, whereas if a bipolar pulse having a voltage Vfs is applied to a predetermined pixel electrode in the planar state, the state is changed to the focalconic state so that the display color is changed from the first color to the second color.
In the cholesteric liquid crystal panel 1, for example, as a bipolar pulse voltage having the voltage value Vps is applied to the whole panel, the whole display area enters the planar state and the displayed information is reset once, thereafter, as a bipolar pulse voltage of the voltage pulse Vfs is applied to a pixel electrode at a necessary position, predetermined information can be displayed and the displayed information can be retained without applying a voltage thereafter.
FIG. 5 is a block diagram showing an example of the structure of a typical liquid crystal driver circuit 21 of related art for driving cholesteric liquid crystal panel 1. Description will be made herein assuming that the cholesteric liquid crystal panel 1 displays n×m pixel information.
A column driver 31 is a driver which is supplied with a clock (CLK) signal and a data (DATA) signal representative of information to be displayed on the cholesteric liquid crystal panel 1, connected to drive voltages ±V2 and GND (0 V), and applies predetermined voltages to column (signal) electrodes Y1 to Yn of the transparent column electrodes 12 of the cholesteric liquid crystal panel 1, at predetermined timings to be described later with reference to FIG. 7.
A row driver 32 is a driver which is supplied with the clock (CLK) signal, connected to drive voltages ±V1 and the same GND as the GND supplied to the column driver 31, and applies predetermined voltages to row (scan) electrodes X1 to Xn of the transparent row electrodes 15 of the cholesteric liquid crystal panel 1, at predetermined timings to be described later with reference to FIG. 7.
The drive voltages V1 and V2 have the voltage values satisfying V1+V2>Vps.
Next, description will be made on a specific example of displaying 3×3, 9 pixels in two colors (two colors, a specific wavelength color and a black color, for example, if the specific wavelength color is green, pixels are displayed in two colors of green and black).
For example, as shown in FIG. 6, description will be made on displaying six pixels (X1, Y1), (X1, Y2), (X2, Y2), (X2, Y3), (X3, Y2) and (X3, Y3) among 3×3, 9 pixels in black and the other pixels in the specific wavelength color. The specific wavelength color is displayed in the state that cholesteric liquid crystal in the planar state makes light of the specific wavelength color be subjected to interference scattering, whereas the black color is displayed by transmission through the transparent cholesteric liquid crystal in the focalconic state.
FIGS. 7 and 8 are timing charts illustrating the operations of the column driver 31 and row driver 32. FIG. 7 is the timing chart illustrating voltages and timings of a bipolar pulse applied to the column electrodes X1 to X3 by the column driver 31 and voltages and timings of a bipolar pulse applied to the row electrodes Y1 to Y3 by the row driver 32, in order to display information of 3×3, 9 pixels shown in FIG. 6. FIG. 8 is a timing chart illustrating bipolar pulses applied across pixel electrodes of (X1, Y1) to (X3, Y3) of 3×3, 9 pixels (across electrodes at cross points of the transparent column electrodes 12 and transparent row electrodes 15), by using the applied voltages described with reference to FIG. 7.
First, in order to reset presently retained information, as shown in FIG. 7 a bipolar pulse of a voltage V1 is applied to the column electrodes Y1 to Y3 and a bipolar pulse of a voltage −V2 is applied to the row electrodes X1 to X3. Therefore, as shown in FIG. 8, a bipolar pulse of (V1+V2) is applied across pixel electrodes corresponding to pixels (X1, Y1) to (X3, Y3). Since V1+V2>Vps, the cholesteric liquid crystal layer 14 between two electrodes, the transparent column electrode 12 and transparent row electrode 15, enters the planar state and makes the specific wavelength light be subjected to interference scattering. Namely, the specific wavelength color is displayed on all pixels (X1, Y1) to (X3, Y3) (hereinafter called all planar reset).
Thereafter, as shown in FIG. 7, the row driver 32 sequentially scans the row electrodes X1, X2 and X3 and applies a bipolar pulse having a voltage V3 to select one of the row electrodes. In correspondence with the select timing of the row electrode, the column driver 31 selectively applies a bipolar pulse −V4 of inverted characteristics to the column electrodes Y1 to Y3. It is assumed herein that V3+V4>Vfs, V1>V3 and V2>V4.
As shown in FIG. 8, a bipolar pulse voltage of V3+V4>Vfs is applied to the six pixels (X1, Y1), (X1, Y2), (X2, Y2), (X2, Y3), (X3, Y2) and (X3, Y3) corresponding to the pixel electrodes of the row and column electrodes to which the bipolar pulses are applied at the same timing. Therefore, the cholesteric liquid crystal layer 14 between two electrodes of the transparent column electrode 12 and transparent row electrode 15 at the corresponding position enters the focalconic state and becomes transparent. Namely, the six pixels (X1, Y1), (X1, Y2), (X2, Y2), (X2, Y3), (X3, Y2) and (X3, Y3) are displayed in black.
Since V3+V4>Vfs and the voltage value Vps is approximately a twofold of the voltage value Vfs, V1+V2>V3+V4 is satisfied.
In this manner, information can be displayed on the cholesteric liquid crystal panel 1 by changing a desired pixel from a specific wavelength color to a black color after all planar reset.